The mobilities of charge carriers in organic semiconductors are at present not more than 1–2 cm2/Vs. The maximum operating frequency of transistors and hence of the electronic switching is limited by the mobility of the charge carriers (holes or electrons). Although mobilities of the order of magnitude of 100 cm2/Vs are sufficient for driver application in the production of TFT active matrix displays, high-frequency applications are generally excluded.
For technical reasons, wireless information transmission (RF-ID systems) can be effected only above a certain minimum frequency. In systems which are supplied directly from the alternating electromagnetic field and hence also have no independent voltage supply, the carrier frequencies 125 kHz and 13.56 MHz are widely used. Fields of use are identification and marking technology (smart cards, ident tags (bar code replacement), electronic postage stamps, etc.).
On the one hand, products which contain a silicon-based chip are too expensive for many applications. For example, a silicon-based ident tag is not suitable for marking foods (price, expiry date, etc.).
On the other hand, polymers or organic semiconductors have the potential that cheap printing techniques can be used for their structuring and application.
However, the mobilities of the charge carriers in these materials are too low for the applications discussed above.
Processes for applying the semiconducting molecules (for example pentacene, oligothiophenes) in a manner which is as ordered as possible, for example by vacuum sublimation, have been developed. Ordered deposition of the organic semiconductor leads to an increase in the crystallinity of the semiconductor material and, owing to an improved π—π overlap between the molecules or the side chains, reduces the energy barrier [1]. These are substantially molecules having a linear structure.
Substitution of the semiconducting molecular units by bulky groups leads to the formation of liquid crystalline-like domains in the deposition of the organic semiconductor from the liquid or gas phase [2].
In the area of polymers, synthesis processes in which as high a regioregularity as possible is achieved by the use of asymmetric monomers have been developed [3].
In [4], various octupolar oligothienyl compounds are described. However, these have only a relatively low charge carrier mobility, in order to enable them to be advantageously used as organic semiconductors and are for the most part irreversibly oxidizable or reducible.
Kotha et al., Synlett 1999, No. 10, 1621–1623, disclose octupolar molecules based on benzene having short thiophene side chains.